1. Field of the Invention
This invention relates to an imaging optical system, and more specifically, to an optical system provided with a variable optical-property element, such as a liquid crystal lens, and an image pick-up device provided with the optical system. Devices for photographing still images and moving pictures with optical apparatuses, such as cameras, microscopes, endoscopes, telescopes, and scanners, come into the category of this image pick-up device.
2. Description of Related Art
In general, focusing of an imaging optical system is performed by moving a lens along the optical axis. On the other hand, an optical system is known which is designed so that a variable focal-length lens, such as a liquid crystal lens, is used and thereby a magnification change and the focusing are performed without moving a lens system.
For such a liquid crystal lens, a polarizing plate is required. In another optical system, a variable focal-length mechanism is realized in which a first liquid crystal lens and a second liquid crystal lens are oppositely arranged so that orientation directions cross at right angles with each other in a plane perpendicular to the optical axis and thereby the polarizing plate is not required.
On the other hand, a diffractive liquid crystal lens is known which is constructed so that a relief pattern of saw-shaped cross section is formed as the liquid crystal lens and energy is concentrated with respect to diffracted light of a particular order of diffraction (diffraction efficiency is optimized).